Rare earth-iron magnet
This is a divisional of application Ser. No. 059,737, filed Jun. 8, 1987, now U.S. Pat. No. 4,863,805. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved permanent magnet and more particularly to a corrosionproofing treatment for rare earth-iron magnet. 
     2. Description of the Prior Art 
     A Sm-Co base magnet has been conventionally used as a high energy product magnet. However, a rare earth-iron base magnet has recently attracted attention because of its low cost, good mechanical processing and higher energy product. Particularly, a magnet having a composition comprising, in terms of atomic %, 8 to 30% R (wherein R is at least one rare earth element including yttrium), 2 to 28% boron with the balance being iron and unavoidable impurities, or 8 to 30% R, 2 to 28% B, more than zero and not exceeding 50% Co, and the balance being Fe with impurities is effective. 
     About the above rare earth-iron magnets are disclosed in EP Nos. 101 552 A2 and 106 948 A2. 
     However, the rare earth-iron base magnet is inferior in corrosion and chemical resistance to the one based on Sm-Co, which requires the treatment of the surface thereof. In fact, an attempt has been made to treat the surface with aluminium chromate. 
     SUMMARY OF THE INVENTION 
     The rare earth-iron base magnet produced by the sintering process is poor in corrosion and chemical resistance, particularly poor in acid and alkali resistance because of the fact that iron is contained therein as a main component. Further, the magnet has a number of voids therein. 
     Therefore, when a wet treatment is conducted for corrosion protection, a chemical for treatment enters the voids of the inside of the magnet and stays there, which brings about the occurrence of corrosion of the inside of the magnet and blistering of the surface coating layer. 
     For example, in the surface treatment such as wet plating, the surface is attacked during pretreatment with acid, alkali or the like, or during plating. Even when the plating is successfully conducted, there occurs internal corrosion and erosion of the void portion among the grains due to an adverse effect of the chemical which has penetrated into the inside. The occurrence of the above-mentioned corrosion leads to the deterioration of the magnetic characteristics. For example, in the case of a rotor magnet for a step motor of a watch, the magnetic characteristics are lowered by about 10% in an early stage as compared with an untreated magnet, and are further lowered with the lapse of time. At the same time, there is caused a problem that the characteristics with respect to mechanical strength are also lowered. 
     It is an object of the invention to provide a rare earth-iron magnet comprising Fe, B and R, or Fe, Co, B and R wherein R is at least one rare earth element including Y, which is excellent in corrosion resistance, magnetic stability and mechanical strength. 
     According to the present invention, there is provided a rare earth-iron magnet, more particularly a sintered rare earth-iron magnet comprising a sealing agent filling the voids among the grains of the magnet and a coating, such as a plating layer, organic coating or plasma-polymerized coating, covering the entire surface of the magnet. 
     The above object and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial sectional view of a magnet embodying the invention. 
     FIG. 2 is a partial sectional view of another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As previously noted, the present invention involves an improved treatment of the surface of a rare earth-iron magnet. 
     In the present invention, the voids among the grains of the magnet are filled with a sealing agent and the entire surface of the magnet is formed of a coating layer, such as a plating layer, organic coating, organic polymer coating or metal-organic polymer composite coating. 
     FIG. 1 is a partial sectional view of an embodiment of the invention. 
     In FIG. 1, the numeral 1 indicates a sintered rare earth-iron magnet. The magnet produced by sintering has void portions among the gains of the magnet. These void portions are filled with a sealing agent 2. 
     Examples of the sealing agent include linseed oil, wax, water glass, polyester resin, phenolic resin, epoxy resin, and anaerobic resin. The sealing agent is poured into the void portion in a liquid state and then cured. 
     The surface of the magnet is formed of a coating layer 3. 
     The invention is illustrated in more detail by the following non-limiting examples: 
     EXAMPLE 1 
     As starting materials, electrolytic iron, boron and neodium of which purity is 99.5% or more were used. The materials were melted by high-frequency melting and then casted by water-cooled copper mold. As a result, an ingot composing 16Nd-8B-76Fe was produced. 
     Next, the ingot was ground into pieces by a stamp mill and a ball mill, so that powder of which fine grit size is 3-10 micron was obtained. The powder was put into a metal mold, oriented in a magnetic field of 15 KOe, and molded by the pressure of 1.5 t/cm 2  in parallel with the magnetic field. The molded product was sintered at 1100° C. for 1 hour in Ar gas and allowed to cool. Further, age processing was conducted at 600° C. for 1 hour. Thus, a permanent magnet was obtained. 
     The permanent magnet obtained was cut down into pieces of which sizes are 20 m/m×10 m/m×5 m/m. Then, grease was removed from the pieces by an organic solvent. Subsequently, the minute voids in the magnet pieces were filled by a pressure reducing method. In order to fill the voids, the above pieces were soaked in a vessel for pressure reduction in which anaerobic adhesive solvent was provided, and left there with the pressure reduced (1 Torr) for 3 minutes. Then, the pressure was reset to the air pressure. Consequently, the voids were filled. The unhardened anaerobic adhesive solvent which is attached to the surface of the pieces were degreased by alkali and cleaned by acid. Subsequently, various kind of plating were conducted under the conditions shown in Table 1. Then, the corrosion resistance, adhesion and magnetic characteristics after plating were evaluated. The result is shown in Table 2. 
     The corrosion resistance is evaluated by the appearance of the sample pieces placed in a room of 60° C. and 90% relative humidity for 240 hours. The adhesion is evaluated by conducting a pulling test by means of adhesive tapes and checking if a thin film layer on the surface is peeled off. In Table 1, the electroless plating sample were prepared by the well-known activating in tin(II) chloride and paradium chloride solutions as a sensibility treatment. 
     The unplated sample having the same element as that of the sample of the present invention undergoes the evaluation on the corrosion resistance and magnetic characteristics in the same method as the above. Also, the result is shown in Table 2 for comparison. 
     
                                           TABLE 1                                 
__________________________________________________________________________
kind of plating        plating conditions                                 
     and the thickness current                                            
                            bath temp.                                    
                                  time                                    
sample                                                                    
     of plating                                                           
              plating bath                                                
                       density                                            
                            (°C.)                                  
                                  (minutes)                               
__________________________________________________________________________
1-1  Cu plating 3μ                                                     
              electroless copper                                          
                       --   20    20                                      
              plating bath                                                
1-2  Cu plating 5μ                                                     
              electroless copper                                          
                       --   20    30                                      
              plating bath                                                
2-1  Cu plating 3μ                                                     
              copper cyanide                                              
                       2 A/dm.sup.2                                       
                            35     6                                      
              plating bath                                                
2-2  Cu plating 5μ                                                     
              copper cyanide                                              
                       2 A/dm.sup.2                                       
                            35    10                                      
              plating bath                                                
3-1  Ni-p plating 3μ                                                   
              electroless nickel                                          
                       --   90     9                                      
              plating bath                                                
3-2  Ni-p plating 5μ                                                   
              electroless nickel                                          
                       --   90    15                                      
              plating bath                                                
4-1  Ni plating 3μ                                                     
              nickel plating                                              
                       2 A/dm.sup.2                                       
                            60     9                                      
              (watt bath)                                                 
4-2  Ni plating 5μ                                                     
              nickel plating                                              
                       2 A/dm.sup.2                                       
                            60    15                                      
              (watt bath)                                                 
5-1  Cu plating 5μ                                                     
              electroless copper                                          
                       --   20    30                                      
     as under layer +                                                     
              plating bath                                                
     Ni plating 1μ                                                     
              nickel plating                                              
                       2 A/dm.sup.2                                       
                            60     3                                      
              (watt bath)                                                 
5-2  Cu plating 5μ                                                     
              copper cyanide                                              
                       2 A/dm.sup.2                                       
                            20    10                                      
     as under layer +                                                     
              plating bath                                                
     Ni plating 1μ                                                     
              nickel plating                                              
                       2 A/dm.sup.2                                       
                            60     3                                      
              (watt bath)                                                 
__________________________________________________________________________
 
    
     
                       TABLE 2                                                     
______________________________________                                    
                 adhesion magnetic                                        
          corrosion                                                       
                 charac-  characteristics                                 
sample      resistance                                                    
                     teristics                                            
                              Br   iHc  (BH)max                           
______________________________________                                    
compari-                                                                  
       unplated X        --     100  100  100                             
son data                                                                  
present                                                                   
       1-1      ○ ⊚                                 
                                99   98   98                              
invention                                                                 
       1-2      ○ ⊚                                 
                                98   99   98                              
       2-1      ○ ⊚                                 
                                99   99   99                              
       2-2      ○ ⊚                                 
                                98   98   99                              
       3-1      ⊚                                          
                         ⊚                                 
                                99   98   99                              
       3-2      ⊚                                          
                         ⊚                                 
                                99   99   98                              
       4-1      ⊚                                          
                         ⊚                                 
                                98   99   99                              
       4-2      ⊚                                          
                         ⊚                                 
                                98   98   99                              
       5-1      ⊚                                          
                         ⊚                                 
                                99   99   98                              
       5-2      ⊚                                          
                         ⊚                                 
                                99   99   98                              
______________________________________                                    
 X: Indicates that a remarkable red rust is generated.                    
  ○ : Indicates that a change of color is hardly generated.        
 ⊚: Indicates that a change of color is not generated at   
 all.                                                                     
 
    
     As is obvious from Table 2, the unplated sample generates remarkable red rust on the surface of the magnet pieces as a result of testing the corrosion resistance. On the other hand, the permanent magnet of the present invention presents sufficient corrosion resistance. Moreover, the surface processing hardly causes a change in the magnetic characteristics. That is, the present invention attains to produce a NdFe magnet having excellent reliability. 
     EXAMPLE 2 
     A rotor of a step motor for a watch, which was prepared from a sintered Nd-Fe-B base magnet, was cleaned through washing with trichloroethylene. Subsequently, the rotor was immersed in an anaerobic adhesive solvent of which the viscosity is adjusted to 10 cp. 
     The rotor immersed in the anaerobic adhesive is placed in a vacuum container (1 Torr) and is allowed to stand in it for 5 min. followed by returning the pressure of the vacuum container to the atmospheric pressure. Thus, the anaerobic adhesive penetrated the voids among the grains of the inside of the rotor and was cured. Thereafter, the anaerobic adhesive remaining uncured was dissolved and washed away with alcohol. A 5 μm-thick organic coating layer comprising poly-p-xylene was formed on the surface of the rotor by the vacuum deposition method, to thereby obtain a final product. The rotor for electronic watches thus obtained was allowed to stand in a thermo-hygrostatic atmosphere kept at a temperature of 40° C. and a relative humidity of 95% for 200 hr. As a result, it was found that the rotor caused neither rusting nor discoloration and had a satisfactory quality for use as a rotor for electronic watches. 
     The rare earth-iron base magnet thus obtained completely prevents the penetration of chemicals, water, etc, thereinto from the outside by virtue of the filling of the voids among the grains with a sealing agent and, at the same time, prevents discoloration and rusting by virtue of the presence of the organic film layer on the surface thereof. 
     According to the present embodiment, the same effect can be also attained with respect to rare earth-iron base magnets other than those described in the above example. 
     Further, with respect to the sealing agent, linseed oil, wax, varnish, water glass, polyester resin, phenolic resin, epoxy resin, etc, can be also employed besides the anaerobic adhesive. Moreover, the organic coating can be also formed using a coating agent such as a fluororesin. 
     In another embodiment of the present invention, the surface of a magnet is formed of a sealing agent filling the voids among the grains of the magnet and a plasma-polymerized coating, such as an organic polymer coating or metal-organic polymer composite coating, covering the entire surface of the magnet. The present invention remarkably enhances the corrosion resistance and long-term stability of magnetic characteristics through a combination of pinhole-freeness of the plasma polymerized coating derived from its high denseness with an effect of filling the void portions among the grains. 
     In the formation of a coating by plasma polymerization, a coating having a dense network structure can be formed through a complicated combination of a process of forming an intermediate of a polymer in a gas phase from a monomer and depositing the intermediate on a substrate to form a polymer coating thereon, a process of polymerizing a monomer, dimer or trimer adsorbed on the surface of a substrate through activation by means of a plasma, with a process of eliminating the formed polymer by means of etching. Therefore, the formed coating is free from pinholes despite the small thickness and, at the same time, is excellent in wear and chemical resistances. Further, since the coating is formed in a gas phase, the formed polymer can be well applied even onto tiny voids. Moreover, a metal-polymer composite coating can be formed by vaporizing a metal when the plasma polymerization is conducted, which enables further improvement in wear resistance. In the present invention, the corrosion resistance, magnetic stability and mechanical strength of the magnet are improved through a combination of the above-mentioned formation of a polymer coating with the treatment for filling the voids among magnet grains, in which a plasma-polymerized coating is formed only with difficulty, with a sealing agent. 
     The above described embodiment will now be described in more detail with reference to the following examples 3 and 4. 
     EXAMPLE 3 
     A rotor of a step motor for a watch, which was prepared from a sintered Nd-Fe-B base magnet was cleansed through with a solvent etc. The rotor was then immersed in an anaerobic resin comprising an acrylic ester, of which the viscosity was adjusted to 10 cp, under a degree of vacuum of 1 Torr for 5 min. The rotor was taken out in air and allowed to stand, thereby curing the resin present in the voids among the grains of the magnet. The uncured resin present on the surface of the magnet was removed with an alcohol. An argon gas and a methyl acrylate monomer were fed using a plasma polymerization device of the internal electrode type. A plasma was generated under 3×10 -1  Torr with a high-frequency power source of 13.56 MHz to conduct plasma polymerization, thereby forming a 0.5 μm-thick polymethyl acrylate coating. 
     The rotor thus formed was entirely coated, including the voids among the grains, with the anaerobic resin and polymethyl acrylate coating. In a thermohygrostatic test conducted at a temperature of 40° C. and a relative humidity of 95% for 100 hr, the rotor exhibited an excellent corrosion resistance without occurrence of corrosion and also exhibited a remarkably excellent long-term stability without causing the lowering in mechanical strength and magnetic characteristics. 
     EXAMPLE 4 
     A sintered Nd-Fe-B base magnet was filled with a sealing agent. A high-frequency ion plating device was evacuated to 5×10 -5  Torr. An argon gas was fed to conduct an ion bombardment. Ethylene monomer was fed, and a plasma polymerization was then conducted under a pressure of 1×10 -3  Torr by applying a high-frequency electric power of 13.56 MHz at 100 W while evaporating aluminum through resistance heating, thereby forming a 0.4 μm-thick aluminum-polyester composite polymer coating in which aluminum was uniformly distributed in a polyester polymer coating. 
     The magnet thus obtained was excellent in corrosion resistance, magnetic stability and mechanical strength like the magnet obtained in EXAMPLE 3. Further, the magnet had an enhanced coating strength and an excellent wear resistance by virtue of the compounding of aluminum. 
     The further embodiments are as follows. In a further embodiment of the present invention, a magnet having an excellent corrosion resistance is provided by filling the voids present in the inside of a magnet produced by sintering with a sealing agent, coating the surface thereof with a coating layer such as plating layer and filling the defective portion of the resulting coating layer and the voids present in the inside of the magnet again with the sealing agent, thereby shielding the magnet material from a corrosive atmosphere. 
     That is, a magnet produced by sintering was subjected to a treatment comprising filling the voids present in the inside of the magnet with a sealing agent by vacuum impregnation or the like, coating the surface of the magnet with a coating layer by wet plating, dry plating or the like and filling the defective portion of the coating layer and void portion present in the inside of the magnet again with the sealing agent. 
     The above-mentioned treatment serves to shield the voids in the inside of the magnet, formed during the production of the magnet, from the outside corrosive atmosphere and chemicals used in the subsequent steps of treatment by virtue of the initial filling of the voids with sealing agent and, therefore, prevents the occurrence of corrosion. 
     The coating layer shields the surface of the magnet from the outside corrosive atmosphere and, therefore, prevents the occurrence of corrosion. 
     Further, the voids of the magnet newly produced during formation of the coating layer made of a sealing agent on the surface of the magnet as well as defective portions of the resulting coating layer are shielded from the outside corrosive atmosphere by the second filling of the sealing agent, which prevents the occurrence of corrosion. 
     It is needless to say that the voids produced during the production of the magnet and independently existing without communicating with the surface of the magnet are free from the occurrence of corrosion even when they are not filled with the sealing agent according to the present embodiment, because they are not exposed to the chemicals used in the formation of the coating and the outside corrosive atmosphere. 
     The further embodiment will now be described in more detail with reference to FIG. 2 and the following examples 5 and 6. 
     The partial sectional view of further embodiment is shown in FIG. 2. Referring to FIG. 2, numeral 4 is a sealing agent packed, after coating layer 3 composed of plating layer, organic coating or plasma-polymerized coating, into defective portions of the coating layer and the voids present in the inside of the magnet. The reference numerals 1 and 2 indicate a magnet produced by sintering and a sealing agent packed, prior to the coating, into the voids present in the inside of the magnet. 
     EXAMPLE 5 
     A Nd-B-Fe base magnet produced by sintering was impregnated in vacuo with a sealing agent comprised mainly of an acrylic ester and was allowed to stand at room temperature for 10 min. The uncured resin present on the surface thereof was removed by dissolving with an organic solvent. 
     Then, the surface of the magnet was sufficiently washed and activated, followed by the formation of an electroless copper plating in a thickness of 5 μm. Subsequently, an electroless nickel plating was formed thereon in a thickness of 2 μm. 
     Finally, the magnet was impregnated in vacuo again with a sealing agent comprised mainly of an acrylic ester and was allowed to stand at room temperature for 10 min. Thereafter, the uncured resin present on the surface thereof was removed by dissolving with an organic solvent. 
     The magnet thus obtained was applied to a thermohygrostatic test (40° C., 95%, 100 hr). In the test, the magnet exhibited an excellent corrosion resistance. (See Table 3.) 
     
                       TABLE 3                                                     
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Results of Corrosion Resistance Test                                      
______________________________________                                    
                magnet in           magnet only                           
       magnet   the form of                                               
                          magnet having                                   
                                    filled with                           
sample to                                                                 
       prepared magnet    only coating                                    
                                    sealing                               
be tested                                                                 
       in Ex. 5 material  layer     agent                                 
______________________________________                                    
results of                                                                
       no       entire    partial   entire                                
thermo-                                                                   
       corrosion                                                          
                corrosion corrosion corrosion                             
hygro-                                                                    
static test                                                               
______________________________________                                    
 
    
     EXAMPLE 6 
     A Nd-B-Fe base magnet produced by sintering was impregnated in vacuo with a sealing agent comprised mainly of an acrylic ester and was allowed to stand at room temperature for 10 min. The uncured resin present on the surface thereof was removed by dissolving with an organic solvent. 
     Subsequently, the magnet was degreased and washed with trichloroethylene, followed by the formation of metallic titanium coating and titanium nitride coating in a thickness of about 3 μm on the surface of the magnet. Prior to the coating, the surface of the magnet was cleaned in a film forming device by ion bombardment through discharge of an argon gas. 
     Finally, a coating of poly-p-xylylene (e.g., Parylene manufactured by Union Carbide Corp.) was formed in a thickness of about 5 μm by vacuum deposition. 
     The magnet thus obtained was applied to a thermohygrostatic test (40° C., 95%, 100 hr). In the test, the magnet exhibited an excellent corrosion resistance. (See Table 4.) 
     
                       TABLE 4                                                     
______________________________________                                    
Results of Corrosion Resistance Test                                      
______________________________________                                    
                     magnet in                                            
                     the form of                                          
                                magnet having                             
sample to                                                                 
        magnet prepared                                                   
                     magnet     only                                      
be tested                                                                 
        in Ex. 6     material   coating layer                             
______________________________________                                    
results of                                                                
        no corrosion entire     partial                                   
thermo-              corrosion  corrosion                                 
hygro-                                                                    
static test                                                               
______________________________________                                    
 
    
     Having described a specific embodiment of our bearing, it is believed obvious that modification and variation of our invention is possible in fight of the above teachings. 
     As mentioned above, the present invention enabled the prevention of the penetration of chemicals and water into the magnet from the outside by virtue of the filling of the voids among the grains of the inside of the rare earth-iron base magnet with a sealing agent and, at the same time, enabled the prevention of corrosion by virtue of the formation of a coating layer on the surface thereof. Further, in the present invention, the magnetic characteristics and mechanical strength of the magnet are not spoiled if chemicals which attack the rare earth-iron material are not used. 
     The term &#34;rare earth-iron magnet&#34; is used herein in a comprehensive sense. That is, the magnet materials may be formed by the melt-quenched amorphous ribbons or sputtered thin films of rare earth-iron alloy. The method of making a rare earth-iron magnet is not restricted to the method shown in example 1. 
     The melt-quenched ribbons being magnetically isotropic by nature are produced in the following steps; making a starting material, melting the starting material by high-frequency melting, quenching the molten material at a rate such that it solidifies substantially instantaneously to form an alloy with a substantially amorphous to very finely crystalline microstructure. The resultant ribbons are comminuted, and compacted or hot-pressed into a magnet shape. On the other hand, the rare earth-iron magnet produced by sintering as shown in example 1 may obtain magnetically anisotropic permanent magnets for the practical purpose. According to the present invention, a sintered magnet having an extremely excellent corrosion resistance can be obtained even though the material of the magnet inherently has a structure full of holes.